Within many fields, there has been a tendency to exchange devices made of glass with devices made of plastic and, in many cases, a large amount of different plastic materials have been tested for use in such devices. Many devices which earlier were prepared from glass are now being prepared from different plastic materials and such devices now made from plastic materials have properties, or at least some properties, which are better than the properties of the analogous devices when made from glass. However, it is not always so that all devices now made from plastic materials have properties which are better than the properties of the analogous devices when made from glass. An example of this is medical containers made from plastic materials such as COP (cyclic olefin polymer) and COC (cyclic olefin copolymer). Unfortunately, medical containers made from plastic materials such as COP or COC are sensitive to non-polar solvents. For example, a COC such as Topas is prone to Environmental Stress Cracking (ESC) at low strain levels, e.g., 0.5%, when in contact with non-polar solvents such as Vaseline or MCT (medium-chain triglycerides) oil, representing ingredients present in many hand lotions. Thus, Topas cannot completely satisfactory be used for a liquid drug container (medical container) when user handling is required, especially not if the medical container is to be used a certain time after the container has been touched by a person having had hand lotion on the fingers and if there are residual stresses in the material or the material is subjected to an external load or strain.
JP 2009 207618 describes a manufacturing method of medical containers used for storing pharmaceutical substances by forming a cover made of parylene (poly-xylylene polymer), fluorine or carbon is formed in inner surface of storage chamber. The adsorption and permeation of medical agent is prevented. Cracking is not dealt with therein, neither is COC.
In Jahrbuch Oberflaechentechnik 64 (2008), 89-99, it is mentioned that, for example, polycarbonate, polyethene, cycloolefin copolymer and polytetrafluoroethylene can be coated with parylene. Some of the advantages mentioned for some of these coated materials are barrier properties, e.g., to oxygen and aqueous vapour, electrical insulation, and high thermal stability; however, cracking is not mentioned. The paper does not deal with medical containers.
Environmental Stress Cracking (ESC) is one of the most common causes of unexpected brittle failure of thermoplastic (especially amorphous) polymers known at present. Environmental stress cracking may account for around 15-30% of all plastic component failures in service.
ESC and polymer resistance to ESC (ESCR) have been studied for several decades. Research shows that the exposure of polymers to liquid chemicals tends to accelerate the crazing process, initiating crazes at stresses that are much lower than the stress causing crazing in air. The action of either a tensile stress or a corrosive liquid alone would not be enough to cause failure, but in ESC the initiation and growth of a crack is caused by the combined action of the stress and a corrosive environmental liquid.
ESC is somewhat different from polymer degradation in that stress cracking does not break polymer bonds. Instead, it breaks the secondary linkages between polymers. These are broken when the mechanical stresses cause minute cracks in the polymer and they propagate rapidly under the harsh environmental conditions. It has also been seen that catastrophic failure under stress can occur due to the attack of a reagent that would not attack the polymer in an unstressed state.